Objective: The rodent-Hymenolepis microstoma system is an excellent model to demonstrate some types of pathological changes which might occur during a host response, since (1) the primary host response (extensive biliary hyperplasia) is easily recognizable at the macroscopic level, (2) the host response is initiated extremely rapidly following infection, and (3) H. microstoma is easily maintained in the laboratory. Although (1) this model system can be easily adapted to the study of biochemical changes which may accompany macro- and microscopical morphological changes in the host, and although these changes may be important in defining more clearly the host-parasite relationships of biliary parasites of medical and economic importance (e.g. Clonorchis sinesis, Opisthorchis sp., Dicrocoelium dendriticum and Fasciola hepatica), the biochemical changes associated with the host- parasite interactions of H. microstoma and the rodent host have received only limited attention. The overall objective of the proposed research is to define these biochemical changes, and correlate these changes with the histopathology associated with infections of this parasite. Background: Although descriptions of the pathology associated with H. microstoma infections in rodents appeared in the literature 46 years ago, it was not until 1961 that detailed studies of the host-parasite relationship of H. microstoma and the rodent host were initiated. Since 1961, some significant contributions dealing directly with the host- parasite relationship of H. microstoma have been made.